Enhanced catalytic activity of Ce1-xMxO2 (M = Ti, Zr, and Hf) solid solution with controlled morphologies

Article abstract of DOI:10.1039/b923217f

Ce_1-xM_xO₂ (M = Ti, Zr, and Hf) nanomaterials with controlled morphology and composition were synthesized by a two-step route for the first time. These nanomaterials exhibit high activities for hydrogen reduction and ethanol reforming reactions, and therefore, they may be useful for applications in catalysis and solid oxide fuel cells.

Full text of DOI:10.1039/b923217f

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Enhanced catalytic activity of Ce M O (M = Ti, Zr, and Hf)
1ꢀx x 2
solid solution with controlled morphologiesw
Wei-Ta Chen,z Kuei-Bo Chen, Ming-Fang Wang, Sheng-Feng Weng,
Chi-Shen Lee* and M. C. Lin
Received (in Cambridge, UK) 5th November 2009, Accepted 1st March 2010
First published as an Advance Article on the web 26th March 2010
DOI: 10.1039/b923217f
Ce1ꢀx
M
x
O
2
(M = Ti, Zr, and Hf) nanomaterials with
synthetic method to control their composition and crystal
morphologies, as shown in Fig. 1. The powder XRD patterns
of Ce1 M O (M = Ti, Zr, and Hf) containing cube-, rod-,
controlled morphology and composition were synthesized by a
two-step route for the first time. These nanomaterials exhibit
high activities for hydrogen reduction and ethanol reforming
reactions, and therefore, they may be useful for applications in
catalysis and solid oxide fuel cells.
ꢀx
x
2
and tube-shaped nanocrystals are shown in Fig. 2a, which
indicate a pure phase with a face-centered-cubic fluorite-type
ꢀ
crystalline structure (Fm3m, No. 225, JCPDS 78-0694).
CeO
2
Rod- and tube-shaped nanocrystals exhibit broadened
diffraction lines, indicating small particle size and poor
crystallinity. All peaks could be indexed and the refinement
of the lattice constants revealed significant deviation in smaller
unit cell parameters for the rod/tube samples, indicative of
structural distortion due to crystal defects and the small ionic
radius of doped metals (Fig. 2b).
Nanomaterials with controlled morphology exhibit interesting
chemical and physical properties due to their exposed
1
–3
facets.
Considerable efforts have been devoted toward
understanding the correlations between the morphology of
nanomaterials and their catalytic activities such as selectivity
of hydrogen from alcohols; isomerization of olefins;
oxygen reduction reactions; conversion of CO, NO
4
x
, and
The morphologies of Ce1ꢀx
M O were investigated by
x 2
–9
hydrocarbons.
Among materials with controlled shapes,
scanning electron microscopy (SEM), high-resolution
transmission electron microscopy (HRTEM) and selected area
electron diffraction (SAED). Fig. 3 shows a summary of
cerium oxide nanocrystals are of particular interest because
of their unique properties in CO oxidation, water-gas shift,
1
0–13
and oxygen storage capacity.
Recently, the synthesis of
HRTEM, SAED, and an element map for Ce1ꢀx
samples. In general, the crystal shapes of Ce1ꢀx
essentially identical to their corresponding ceria nanocrystals
(Fig. S1 and S2w); and the doped metal ions are distributed
homogenously within the ceria host structure. Cubic products
x 2
M O
metal-doped ceria nanomaterials has attracted considerable
attention because such materials have enhanced activities in
CO oxidation, three-way catalysis, and reduced operating
x 2
M O are
1
4–16
temperature in the solid oxide fuel cell.
on doped ceria indicated that the {100}, {110}, and {111}
surfaces of Ce1ꢀx (M = Ti, Zr, Hf, Pd) affected the
Theoretical studies
of Ce1ꢀx
x 2
M O have a uniform shape with sharp edges and
M
x
O
2
sizes ranging from 30–100 nm. The HRTEM image and SAED
pattern for a cubic nanocrystal indicate a single-crystalline
structure with fourfold symmetry and an interplanar d-spacing
of ca. 0.27 nm for Ti-, Zr-, and Hf-doped samples, which are
near the {002} planes of ceria. The average diameter and
oxidation reactions of CO and methane; however, this has not
yet been confirmed experimentally. Several doped ceria nano-
materials with controlled morphology have been studied
1
7–23
previously.
However, the synthesis of metal-doped ceria
solid solutions with controlled crystalline morphology and
composition remains a significant challenge. Herein, we report
a new method to control the shape and composition of a
Ce1 M O (M = Ti, Zr, and Hf) solid solution. This simple
length of Ce1ꢀx
x 2
M O nanorods are in a range from 20–30
and 100–200 nm, respectively. The interplanar d-spacing
(B0.19 nm) attributed to the {220} planes was observed,
indicating preferential growth along the [110] direction, as
confirmed by the SAED pattern of the h100i zone-axis. These
ꢀx
x
2
and high-throughput synthetic route should provide an
opportunity to study the properties that are affected by the
shape and composition of metal oxide nanomaterials. The
catalytic activities of the metal-doped ceria were measured and
correlated with the morphology and composition of the particles.
results indicate that the Ce1ꢀx
x 2
M O nanorods are enclosed
within the {110} and {100} planes. The average length of
x 2
We synthesized Ce1ꢀxM O (M = Ti, Zr, and Hf) nano-
materials under hydrothermal conditions using a two-step
Department of Applied Chemistry, National Chiao Tung University,
1001 University Road, Hsinchu 30010, Taiwan.
E-mail: chishen@mail.nctu.edu.tw; Fax: +886-3-5723764
w Electronic supplementary information (ESI) available: Experimental
details, SEM/TEM; TPR, BET measurement; hydrogen selectivity and
Fig. 1 Schematic illustration of the two-step route to synthesize
elemental analysis for Ce1ꢀx
0.1039/b923217f
z W.-T. Chen and K.-B. Chen contributed equally to this work.
x 2
M O (M = Ti, Zr, Hf). See DOI:
x 2
Ce1ꢀxM O (M = Ti, Zr, and Hf). The precursor of the ceria
1
nanotube contains a small amount of Zr (B1%, ref. 23).
3
286 | Chem. Commun., 2010, 46, 3286–3288
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The chemical compositions of the obtained Ce1ꢀx
x 2
M O
nanomaterials have been characterized by energy dispersive
spectrometry (EDS) and inductively coupled plasma-mass
spectrometry (ICP-MS), and the results are summarized in
Table 1. In general, the amount of doped metal in rod- or
tube-shaped samples is larger than that in the cube-shaped
4
+
sample, and the amount of doped Zr is higher than those of
4
+
4+
Ti and Hf for all Ce1ꢀx
M
x
O
2
samples due to the similar
4+
4
+
˚
(0.98 A) and Ce
˚
(1.11 A). The
Fig. 2 (a) X-ray diffraction pattern of Ce1ꢀx
M O
x 2
samples; the
ionic radius of Zr
indexes of X-ray features are labeled. Labels A, B, and C denote
maximum amount of doped metal ions was observed in
Zr-doped ceria nanorods (34 at% Zr); the minimum value of
cube, rod, and tube; 1, 2, and 3 denote Ti-, Zr-, and Hf-doped samples,
x 2
respectively. (b) Refined lattice constant of Ce1ꢀxM O (M = Ce, Ti,
2
2% in Zr-doped nanocubes is considerably larger than the
Zr, and Hf).
maximum value in Ti- and Hf-doped nanocubes. According to
the BET measurements, the ratio of the average surface area
for (rod, tube)/cube are 5.3, 5.6, and 4.0 for Ti-, Zr-, and
Hf-doped samples, respectively; these ratios are considerably
larger than those for the average atomic concentration of
doped metal 1.6, 1.5, and 4.4 for Ti-, Zr-, and Hf-doped
samples, respectively. The results of surface area and elemental
analyses indicate that the difference in the amount of doped
metal for the nanocube ceria solid solution as against the
nanorod/nanotube one was directly related to the initial
concentration of doped metal ions, temperature, exposed
Ce1 M O2 (M = Ti, Zr, and Hf) nanotubes is several
x
ꢀx
hundred nanometres, and the inner diameter and wall
thickness is in the range of 30–50 and 10–20 nm, respectively.
Interestingly, the SAED pattern of a Hf-tube indicated a
single-crystalline structure with preferential growth along
the [110] direction (tube axis) in a manner similar to that in
rod samples. However, the SAED patterns and HRTEM
images of the Ti and Zr samples suggested polycrystalline
products.
The specific surface areas (Barrett-Emmett-Teller method,
facets, and specific surface areas of CeO
2
nanocrystals.
Fig. S3w) for Ce1 M O (M = Ti, Zr, Hf) decrease gradually
ꢀx
as compared to those of pure ceria. For cubic Ce1 M O , the
x
2
According to the results, we propose the following steps for
controlling a solid solution of Ce1 M O nanomaterials: first,
ꢀx
specific surface area values are similar to those of pure ceria
x
2
ꢀx
x
2
CeO2 nanocrystals with the specific shape crystal were
prepared by hydrothermal conditions; the particle size
difference between cube and rod/tube is manifested in the
2
ꢀ1
nanocubes (B15.3 m g ); this may be attributable to the
large size and high crystallinity of the fine nanoparticles. The
average surface areas of nanorod and nanotube samples are
2
surface area. Second, the as-synthesized CeO nanomaterials
2
ꢀ1
6
9.5 and 43.3 m
g
, respectively; these are considerably
were used as the reagents in another hydrothermal process
with the precursor of transition metals to prepare ceria solid
solution. The amount of doped metal is limited and different
for Ti, Zr, and Hf metal elements. The possible mechanism
2
ꢀ1
larger than that of nanocube samples (11.5 m g ), which
could be associated with an increase in the oxygen vacancies
on the surface.
4
+
may be described by the different exchange rate of M ion on
110} and {100} facets and the formation of the nanostructure
{
depends strongly on the precursor, and we attribute this varied
amount of doped metal to their different surface areas among
4
,23,24
cubic and rod/tube shapes of nanomaterials.
The as-synthesized Ce1ꢀx solid solution was studied
from the viewpoint of hydrogen selectivity of ethanol reforming
H2); this is an important step in the production of hydrogen
from a renewable energy resource. We placed catalysts of
-wt% Ru/Ce1ꢀx supported on an alumina substrate
x 2
M O
(
S
5
x 2
M O
and measured the hydrogen selectivity to ethanol. The overall
reaction is described as follows:
1
2
C
2
5
H OH + 2H
2
O + O
2
- 2CO
2
+ 5H
2
Table 1 Summary of elemental analysis for Ce1ꢀx
M
x
O
2
(M = Ti, Zr, Hf)
Hf
M
Ti
Zr
Cube
SEM-EDS
TEM-EDS
ICP-MS
SEM-EDS
TEM-EDS
ICP-MS
SEM-EDS
TEM-EDS
ICP-MS
16.2%
13.3%
20.7%
29.9%
27.2%
27.6%
21.8%
19.3%
24.2%
22.0%
19.1%
20.8%
34.5%
34.6%
29.4%
32.7%
31.2%
29.4%
4.7%
4.1%
4.1%
20.1%
18.9%
21.7%
21.5%
19.4%
22.3%
Rod
x 2
Fig. 3 HRTEM images of Ce1ꢀxM O (M = Ti, Zr, and Hf)
samples. The insets of each figure show magnified TEM images, an
element map of Ce and M (M = Ti, Zr, and Hf), and SAED patterns.
The labels are the same as those in Fig. 2.
Tube
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We thank Prof. H.-T. Chu for the BET measurements and
Prof. Ikai Wang (NTHU) for the TPR experiments. The
Institute of Nuclear Energy Research (contract NL940251),
MOE ATU program, and National Science Council (contracts
NSC 94-2120-M-009-014, 98-2113-M-009-007-MY3) supported
this research. MCL acknowledges the Taiwan Semi-
conductor Manufacturing Co. for the TSMC Distinguished
Professorship and the Taiwan National Science Council for
the Distinguished Visiting Professorship at the National Chiao
Tung University.
Fig. 4 (a) Hydrogen selectivity over catalysts with Ce1ꢀx
samples with different morphologies. (b) Hydrogen consumption rate
observed from TPR profiles over Ce1ꢀx samples.
x 2
M O
x 2
M O
The catalytic activity was evaluated in terms of hydrogen
selectivity (SH2), which is defined as the molar ratio of the
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x 2
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2
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synthesized for the first time by a two-step method. The
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3
288 | Chem. Commun., 2010, 46, 3286–3288
This journal is ꢁc The Royal Society of Chemistry 2010